Apparatus for delivering electrical stimulation energy to body-implanted apparatus with signal-receiving means

ABSTRACT

An apparatus for delivering electrical stimulation energy to body-implanted apparatus with signal-receiving means is described which is particularly useful in implanted devices, including nerve and muscle stimulators, artificial organs, and certain electro-mechanical devices such as implanted dispensers for the administration of fluids to the body system. The electrical portion of the system includes a transmitter and receiver, with the transmitter normally being disposed or worn externally of the body of the patient or user and being arranged to generate pulses of predetermined amplitude at radio frequencies. The receiver includes a pick-up, a receiver and an output electrode arrangement which is normally subcutaneously implanted within the body of the patient. Since variations in signal energy received by the receiver from its coupled transmitter may occur due to relative positioning of the transmitter-receiver pair, the receiver serves to regulate the amount of energy delivered to the output electrodes such that relatively uniform impulses are normally applied to the output electrodes. Thus, even though there may be variations in the energy coupled from the transmitter to the receiver, a uniform output occurs; the arrangement providing a means for coupling an electrical signal of variable magnitude to provide an output or stimulation signal of constant energy amplitude or magnitude to an implanted device.

Unite States Patent [191 Hagfors 1 APPARATUS FOR DELIVERING ELECTRICAL STIMULATION ENERGY TO BODY-IMPLANTED APPARATUS WITH SIGNAL-RECEIVING MEANS [76] Inventor: Norman R. Hagfors, 4414 TylerSt.

N.E., Minneapolis, Minn. 55421 [22] Filed: July 7, 1971 [21] Appl. No.: 160,368

[52] US. Cl. 128/421, 128/419 C [51] Int. Cl A6ln 1/36 [58] Field of Search 128/419, 421, 422, 2.1 P

[56] References Cited UNITED STATES PATENTS 3,521,087 7/1970 Lombardi 128/2.1 P

3,648,708 3/1972 Haeri 4 128/422 3,311,111 3/1967 B0wers..... 128/419 P 2,532,788 12/1950 Sarnoff 128/421 2,771,554 11/1956 Gratzl 128/421 3,547,127 12/1970 Anderson 128/421 FOREIGN PATENTS OR APPLICATIONS 1,444,363 5/1966 France 128/419 P OTHER PUBLICATIONS Holcomb et al., Medical & Biological Engineering Vol. 7, No. 5, Sept. 1969, pp. 493-499.

Parsonnet et al., Surgical Forum, 1966, pp. 125-127.

Cammilli et al., Annals of the New York Academy of Science, Vol. 111, Art. 3, pp. 1007-1029, June 11, 1964 (pp. 1007-1013 relied on).

[ Mar. 12, 1974 Primary Examiner-William E. Kamm [57] ABSTRACT An apparatus for delivering electrical stimulation energy to body-implanted apparatus with signalreceiving means is described which is particularly useful in implanted devices, including nerve and muscle stimulators, artificial organs, and certain electromechanical devices such as implanted dispensers for the administration of fluids to the body system. The electrical portion of the system includes a transmitter and receiver, with the transmitter normally being disposed or worn externally of the body of the patient or user and being arranged to generate pulses of predetermined amplitude at radio frequencies. The receiver includes a pick-up, a receiver and an output electrode arrangement which is normally subcutaneously implanted within the body of the patient. Since variations in signal energy received by the receiver from its coupled transmitter may occur due to relative positioning of the transmitter-receiver pair, the receiver serves to regulate the amount of energy delivered to the output electrodes such that relatively uniform impulses are normally applied to the output electrodes. Thus, even though there may be variations in the energy coupled from the transmitter to the'receiver, a uniform output occurs; the arrangement providing a means for coupling an electrical signal of variable magnitude to provide an output or stimulation signal of constant energy amplitude or magnitude to an implanted device.

3 Claims, 9 Drawing Figures PAIENTEB m i 2 i974 SHEEF 1 OF 2 l/VVE/VTOR Norman R. Hagfors ZTTO?/VEY PATENTEBHAMZIQM (17952 1,

' SIM! 2 OF 2 INVENTOR Norman R. Hagfors A 7' TOR/V5 Y APPARATUS FOR DELIVERING ELECTRICAL STIMULATION ENERGY TO BODY-IMPLANTED APPARATUS WITI-I SIGNAL-RECEIVING MEANS BACKGROUND OF THE INVENTION Implanted devices have been used for some time for accomplishing a variety of purposes. For example, implanted nerve and muscle stimulators have been used for a substantial period of time, as have artificial organs. In order to control these implanted devices, an electrical signal is normally obtained with a transmitter, which is coupled to an implanted receiver, with the receiver having means for delivering an output or stimulation signal to one or more output electrodes. By way of a specific example, it has been known that relief of pain associated with angina pectoris may be achieved in certain patients through the manual stimulation of the carotid sinus, the stimulation resulting in a general reduction in the peripheral vascular resistance throughout the body, a significant drop in arterial pressure, decreased cardiac rate, and diminished myoear'dical contractility. A reduction in these factors results in a corresponding reduction in the cardiac workload and the myocardial oxygen requirements. Recently, however, it has been found that these results may be achieved through the direct electrical stimulation of the carotid sinus nerve bundle. The apparatus of the present invention is particularly desirable for application to this type of stimulation.

The present invention provides a system for delivering uniform impulses of electrical energy to an output which may be coupled to a nerve bundle, a muscle tissue, or to an artificial organ. The input is derived from a source disposed externally to the body of the patient which is coupled electrically to a surgically implanted radio receiver and an associated electrode or electrodes which are disposed within the body. In other words, a substantially uniform energy envelope is delivered to the output of the system. The transmitter includes a pulse wave form generator, preferably batterypowered and adapted to deliver radio frequency pulses to an antenna. The amplitude of the electrical signal available to the receiver is variable because of the shifting of the relative positions of the transmitter-receiver pair.

In one typical application, a subcutaneously implanted receiver containing one or more electrically isolated receiving coils and circuits is arranged to be operatively coupled to a transmitter and is arranged to supply impulses to an appropriate nerve bundle by means of an implanted electrode terminal. The antenna coil ofthe transmitter is normally placed on or adjacent the skin of the patient at a position disposed directly over the coils of the receiving antenna contained within the implanted receiver. Electrical impulses from the transmitter are thereby inductively coupled through the skin of the patient and converted to a stimulation impulse having a uniform energy envelope, with constant characteristics of amplitude, pulse duration and frequency. The transmitter is preferably and normally designed to permit the physician to regulate the pulse rate and amplitude to a predetermined desired level at the time of implant, and preferably on occasion thereafter.

With the transmitter and transmitter coil located externally, it is apparent that it is difficult to apply electrical pulses of constant energy to the coils of the receiving antenna and ultimately to the stimulation electrodes of the receiver. Such problems do not normally occur when a direct electrical connection is made between the pulse source and the stimulating electrodes, however, such direct electrical connection is not always practical or possible. For example, in the system SUMMARY OF THE INVENTION In accordance with the features of the present invention, circuit means are provided in an implanted receiver for establishing an energy envelope of constant characteristics at the output electrodes. In this connection, the energy envelope will provide an output with constant amplitude to the stimulation electrodes irrespective (within pre-determined margins) of the relative location of the antenna of the transmitter and the coil or coils of the receiver so that the amount of energy delivered will be regulated. In this arrangement, the amount of energy delivered may be regulated by control of the amplitude, pulse duration, or frequency.

Another feature of the apparatus of the present invention involves the design of the receiver to permit the voltage amplitude to be adjusted from a location externally of the body of the patient. This feature may be accomplished by an electrical bypass mechanism which can eliminate the regulator portion of the receiver, thus allowing for appropriate adjustment of the external source. As an alternative, a magnetically adjustable potentiometer may be provided in the receiver circuit to permit variation of the voltage output level of the receiver at the output or stimulation electrodes. Either a permanent magnet or an electromagnet may be em ployed to accomplish the adjustment operation. In still another alternate arrangement, a percutaneously adjustable potentiometer may be employed.

It is accordingly a principle object of the present invention to provide an improved apparatus for applying electrical stimulation energy to body-implanted apparatus with signal-receiving means.

It is a further object of the present invention to provide an improved apparatus for applying electrical stimulation energy to implanted devices, including nerve and muscle stimulators, artificial organs, and certain electromechanical devices.

It is still a further object of the present invention to provide a system for delivering electrical stimulation energy to body-implanted apparatus utilizing an implanted receiver coupled to an externally worn transmitter, the apparatus being arranged to deliver an energy envelope of constant characteristic to the output electrodes of the receiver, this being achieved irrespective of the precise location of the externally worn transmitter.

It is yet a further object of the present invention to provide an apparatus for delivering electrical stimulation energy to body-implanted apparatus with signal receiving means, the implanted receiver being electrically coupled to an external transmitter, the receiver being provided with means for controlably varying the electrical characteristics of the receiver output from a point external to the body.

These and other objects of the invention will become apparent to those skilled in the art upon a reading of the following detailed description of the various embodiments in light of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates the manner in which one embodiment of the present invention may be utilized in an application to stimulate the carotid sinus nerve bundle, this figure showing the system as implanted in a human being;

FIG. 2 illustrates the electrodes and receiver portion of the embodiment of the invention as shown in FIG.

FIG. 3 illustrates a typical transmitter enclosure and associated antenna coil as utilized in the apparatus of FIG. l, with the coil being shown removed from the transmitter enclosure;

FIG. 4 illustrates, by means of an electrical schematic diagram, a preferred embodiment of the receiver portion of the system;

FIG. 5 illustrates the wave forms existing at various points within the circuit of FIG. 4;

FIGS. 6 and 7 illustrate alternative arrangements for producing a constant amplitude output from the receiver in spite of input variations;

FIG. 8 illustrates alternative embodiments whereby the constant current circuitry can be switched out providing all controls accessible from a point remote from the receiver package itself; and

FIG. 9 illustrates a further alternative embodiment of a constant current regulator which is adapted to function in conjunction with a radio frequency receiver.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. I, there is shown a patient Ml having a subcutaneously implanted apparatus for direct stimulation of predetermined nerve bundles. Specifically, there is shown a radio frequency impulse receiver 12 whose output is coupled by way of electrodes 14 and 16 to the carotid sinus nerve bundles l8 and 20of the patient l0.

Electrical stimulation energy passed along the carotid sinus nerves proceeds generally centrally to the medullary cardiovascular centers. It has been found that the affect achieved is a decrease in the peripheral sympathetic stimuli to the arterioles, reflecting in a lowering of blood pressure and a decrease in sympathetic stimuli to the heart. In other applications, electrical stimulation energy may be used for achieving entirely different affects, it being understood that this application of the system is being shown for purposes of explaining the various concepts of the present invention.

Referring now to FIG. 2, it can be seen that the electrodes l4 and I16 terminate distally in cradle-shaped, polished-platinum stimulation electrode structure 22, molded within a silicone rubber head 24 and arranged to make direct physical contact with a nerve bundle when in place, for example, the carotid sinus nerve bundle. Flaps 26 extend from the head 24 and are normally sutured closed during the implant operation to hold the conductive portions of the electrode 22 in permanent contact with the nerve bundle. When installed, only the platinum electrodes and the silicone rubber of the electrode head come into physical contact with the nerve bundle.

Referring again to FIG. 1, the nerve stimulating apparatus of this invention further includes a transmitter or source of radio frequency impulses 38 which may be affixed to the body of the patient by an convenient means such as a belt 30. Alternatively, the transmitter 38 may be carried by the patient in this shirt pocket, or any other convenient location on or adjacent the body. The transmitter assembly 38 consists of a batterypowered pulse generator and an attached antenna coil 32. The coil 32 is connected to the transmitter 38 by way of conductive wires encased or encapsulated within a flexible tube or conduit 34.

As is indicated by the broken line 36 in FIG. I, in use, the transmitter antenna coil 32 is placed on the skin directly over the receiver 12 such that pulses of radio frequency energy are inductively coupled through the skin of the patient from transmitter antenna coil 32 to the receiving coils (not shown in FIG. 1) located within the surgically implanted receiver package 12.

As is shown in FIG. 3, the transmitter is provided with controls which can be adjusted to fix the characteristics of the energy envelope, including such parameters as amplitude, pulse duration, and frequency of the impulses to be produced by the transmitter for required or optimum response.

With continued attention being directed to FIG. 3, the transmitter circuits are enclosed within a rugged, light weight container 38a, the container preferably being sealed to prevent ingress of moisture, dirt, or other contamination. Within this container is a power source in the form of a battery 40.

The cable 34 is adapted to be connected to the transmitter 38 by means ofa connector 44 which mates with a socket 46 on the transmitter case. The transmitter is also provided with an on-off switch 48 which is accessible to the patient so that he can switch the transmitter into operation when a need is indicated, such as upon the onset of angina] pain or prophylactically when he anticipates that a given activity will precipitate an anginal attack.

Referring again to FIG. 2, the implantable portion of the nerve stimulating apparatus of this invention consists of a wafer-like receiver package 12. The receiver electrical components 50 are preferably embeded in epoxy resin, with the entire assembly being encased in an inert medium such as transparent silicone rubber or the like.

With the physical construction of the transmitter, transmitting antenna, receiver and electrodes having been described, attention will now be given to the various circuits which may be included within the receiver package 12 and which function to provide impulses with an energy envelope of constant characteristics, with the impulse energy being regulated through control of the pulse amplitude, the pulse duration, or the pulse frequency delivered to the stimulation electrodes which are in turn coupled to the nerve bundle. Referring to FIG. 4, there is shown a tuned circuit 66 comprising an inductor 68 and a capacitor 70. The component values of the inductor and capacitor are such that the circuit resonates at the transmitter frequency. Coupled to the output of the tuned circuit 66 at the terminal 71 is a diode detector network including a semiconductor diode 72 and a filter capacitor 74. The output of the diode detector appearing at the junction 76 is coupled through a current limiting diode 78 and a coupling capacitor 80 to the output terminals 82, 84 to which the conductors 54 (FIG. 2) are attached. A load resistor 86 is connected to the junction 88 formed between the current limiting diode 78 and the coupling capacitor 80.

FIG. 5 illustrates the wave form of the signals occurring at various points in the circuit of FIG. 4 when the transmitter antenna coil is placed in close proximity to the coil 68 of the receiver. Specifically, the wave form of FIG. 5, waveform a shows the form of the output appearing at the junction 71. The radio frequency signal when applied to the diode detector 72 causes rectification to take place so that only the positive going portion of the envelope results at the junction 76. The capacitor 74 acts as a filter capacitor or band-pass filter for smoothing the signal and removing the RF components of the signal. The diode 78 is a constant current diode of a pre-selected value. Its function is to regulate the current flow, providing a desired current flow independent of the input signal as long as the input signal is above the threshold value of the diode 78. The dotted line in the wave form shown at b in FIG. 5 illustrates this threshold level and the wave form shown at c in FIG. 5 results at the junction 88. The wave form d of FIG. 5 illustrates the pulse which is AC coupled to the electrode load. The electrode assembly can be considered as a resistive element but in practice, it does not exhibit some degree of capacitance causing the voltage wave forms to differ slightly from those illustrated in FIG. 5.

When using a direct wire connection which passes through the skin of the patient from a transmitter to the nerve rather than inductive coupling as in this invention, it is relatively simple to maintain a constant energy pulse for application to the nerve bundle. When the coupling is inductive in nature, however, such as is the case where the pulse generator is external to the body and the receiver and stimulation electrodes are implanted, it is more difficult to maintain or predict accurately the amount of energy being transferred. This is particularly true when the distance between the transmitting antenna coil and the receiving antenna coil varies or a concentricity displacement occurs. The use of the constant current diode 78 in the receiver of FIG. 4 obviates this problem. Specifically, by supplying an excess of energy to the receiving coil and limiting the receiver output with the constant current diode 78, the amount of energy transferred is maintained at a constant value which can be controlled by varying the pulse duration periods.

As an alternate structure, thecurrent limiting diode 78 as illustrated in FIG. 4 may be replaced with a field effect transistor and associated bias potentiometer. In its operation, the field effect transistor functions as a current regulator with the current flow being determined by the bias voltage existing between the source and gate electrodes of the field effect transistor. The voltage of the field effect transistor is independent of the voltage signal appliedto its input so long as the input voltage exceeds the sum of the voltage drop across the associated bias potentiometer and the voltage drop across the load being applied to the terminals,

such as the load terminals 82 and 84. of FIG. 4. The output current can, of course, be adjusted in view of the provision of the bias potentiometer.

In another embodiment, FIG. 7 illustrates a receiver suitable for use in the nerve stimulating system of this invention, wherein the output signal level is controlled through the use of a series type voltage regulator. Again, those components having similar function to the components in FIG. 4 are given identical identifying numerals in FIG. 7. The circuit if FIG. 7 is substantially identical to that of FIG. 4 except that the current limiting diode 78 of FIG. 4 is replaced by a conventional NPN transistor 102 connected in an emitter follower configuration. Specifically, the transistor 102 includes a base electrode 104, an emitter electrode 106, and a collector electrode 108. The collector electrode is connected to the junction point 76 which is the output from the diode detector network. The emitter electrode 106 is directly connected to the junction 88. A voltage divider including a resistor I10 and a potentiometer 112 is connected in parallel with the output of the detector network. Connected in parallel with the resistive element of the potentiometer is a Zener diode 114. This diode serves to maintain the junction 116 at a constant voltage. By adjusting the wiper arm 118 with respect to the potentiometer resistance 112, the bias applied to the base electrode 104 of the transistor can be varied. Of course, variation of this bias affects the resistance presented between the emitter and collector electrodes of the transistor 102 and therefore the output signal amplitude appearing across the terminals 82, 84

A further improvement in the receiver circuit is illustrated in FIG. 8. The circuit of FIG. 8 is identical to that of FIG. 4 except that a normally open magnetically actuated reed type switch is connected in parallel with the constant current diode 78. This switch is identified in FIG. 8 by numeral 120. The switch is placed in the receiver circuitry in such a manner so as to remove the current limiting circuitry, thereby allowing full control of the amplitude and pulse width parameters from the external transmitter without the benefit of the receiver regulating circuit. Because the receiver package is located from one to two centimeters beneath the surface of the skin of the patient, the switch can be actuated by a permanent magnet positioned on the surface of the skin in proximity to the location of the implanted receiver.

In the embodiments of FIG. 6 it is also possible to provide means for adjusting the voltage or current limiting circuitry of the'implanted receiver from an external location without surgical intervention. Specifically, the positionable wiper arms on the potentiometers used in the circuit of FIG. 6 may be moved by means of an external magnet. That is, by coupling the w'iper arm of the potentiometer directly or through a gear reduction to a permanent magnet 01, it is possible to drive the assembly by means of an external magnet (not shown). Alternatively, by coupling the potentiometer wiper arm to a ratchet, which in turn, is coupled directly to a permanent magnet, it is possible to adjust the resistance value of the potentiometer by electro-magnetic pulses which cause the ratchet to rotate.

Still additional alternatives are available by the use of a potentiometer adjustable by a percutaneous needle, or the like. A mechanical pressure ratchet may also be used.

Attention is now directed to FIG. 9 of the drawings wherein a constant current regulator is shown in combination with the radio frequency receiver, and having certain modified features from those shown in the circuitry of FIGS. 4-8. A receiving coil is provided having segments 122 and 124 center tapped at 126. Capacitor 128 is used to tune the receiver to resonance at the carrier frequency. Diodes 130 and 132 are provided in such a manner so as to provide full-wave rectification as an alternate to the half-wave rectification provided On previous illustrations. This feature is provided in order to achieve greater power transfer efficiency. Capacitor 134 functions as a filter capacitor to remove the carrier frequency from the stimulus signal. Variable resistor 136 is provided along with constant current diode 138 to form the reference standard for the regulator device. The current flowing through resistance element 136 is held constant by constant current diode 138, and adjustment of the value of resistance element 136 will adjust the reference voltage developed across resistance element 136. The voltage reference is converted into a current reference by use of resistor 140, resistor 140 having a large ohmic value. Transistor 142 is uti lized as a current amplifier with the collector current applied to the load being approximately equal to the base current, multiplied by the amplification factor B Beta, of the transistor. Capacitor 144 is utilized to block the DC component of the stimulus signal while resistor 146 provides a discharge path for capacitor 144 between pulses. The circuitry as shown in FIG. 9 minimizes variations in output delivered to electrodes 148 and 150 due to changes in input signal as well as load impedance. The stimulus current may be readily adjusted by controlling the resistance value of variable resistor 136.

ln addition to application as a carotid sinus nerve stimulator, the concept of the present invention is readily adaptable for use in electroanalgesia. In particular, one such area of electroanalgesia may involve dorsal column stimulators which stimulators may include an implanted dorsal column electrode and receiver system. The control of the amplitude of the signal being delivered to the electrodes may be controlled in the manner illustrated hereinabove, with the same electrical principles being applied. Other specific applications includes the use as a phrenic nerve stimulator, cardiac pacing, or peripheral nerve stimulation. The apparatus may find further application as a muscle stimulator, such as in bladder stimulation, or the like. The apparatus may also find application as a brain stimulator. For electro-mechanical applications, the device may be used to controlably inject quantities of a particular fluid into the system, such as insulin or the like.

The apparatus of the present invention is arranged to deliver electrical stimulation energy to body-implanted apparatus with an energy envelope of constant characteristic. As such, the range of applications are wide and varied.

As used herein, the stimulation electrodes may be applied to either an animate or inanimate structure. When used for nerve or muscle stimulation, the stimulation or electrodes are coupled to an animate object,

however when coupled to an implanted electro-' mechanical device, the stimulation electrodes will be coupled to an inanimate object. The definition of the term stimulation electrode will, therefore, be readily comprehended and understood.

Thus, it can be seen that there is provided by this invention a means whereby pulses which are accurately controlled in amplitude, duration and frequency can be applied to a nerve bundle within the body of a patient by inductively coupling electrical energy from an externally located transmitter to a subcutaneously located receiver and electrode assembly. Variations of this invention will occur to those skilled in the art upon a reading of the specification and accordingly, the scope of the invention is to be determined by the appended claims.

I claim:

1. Apparatus for applying electrical stimulation signals of a constant energy envelope to an object located within the human body comprising:

a. a transmitter of pulse modulated radio frequency signals; 7

b. a radio receiver network means adaptedto be subcutaneously implanted in the body of a patient and responsive to said pulse modulated signals for producing a demodulated output signal across first and second junctions thereof;

c. a cource of reference potential connected between said first and second junctions;

d. a controllable impedance means having an input coupled to said first and second junctions, output terminals, and a control terminal connected to said source of reference potential for maintaining the voltage at said output terminal substantially constant over a range of variations of said demodulated output signals; and

e. stimulation electrodes adapted to be surgically implanted and electrically coupled to said output terminals.

2. Apparatus as in claim 1 wherein said controllable impedance means comprises a semiconductor current control means having input, output and control electrodes, said input electrode being connected to said first junction, said output electrode being connected through a resistor to said second junction, and said control electrode being coupled to said source of reference potential.

3. Apparatus as in claim 2 and further including magnetically actuatable means for varying said source of reference potential from a location outside of the body. =1 l =l l UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 796, 221 Dated March 12 1974 1nventor(s) Norman R. Hagfors It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4, line 50, "embeded" should read embedded Column 6, line 49, after "FIG. 6" insert end 7 Line 54, after "FIG. 6" insert and 7 Column 8, line 33, "cource" should read source Signed and sealed this 17th day of September 1974.

(SEAL) Attest: I

McCOY M. GIBSON JR. C. MARSHALL DANN Commissioner of Patents Attesting Officer- USCOMM-DC 60376-P69 e u.s. GOVERNMENT PRINTING OFFICE: 1969 o-ass-su F ORM PO-IOSO (10-69) 

1. Apparatus for applying electrical stimulation signals of a constant energy envelope to an object located within the human body comprising: a. a transmitter of pulse modulated radio frequency signals; b. a radio receiver network means adapted to be subcutaneously implanted in the body of a patient and responsive to said pulse modulated signals for producing a demodulated output signal across first and second junctions thereof; c. a cource of reference potential connected between said first and second junctions; d. a controllable impedance means having an input coupled to said first and second junctions, output terminals, and a control terminal connected to said source of reference potential for maintaining the voltage at said output terminal substantially constant over a range of variations of said demodulated output signals; and e. stimulation electrodes adapted to be surgically implanted and electrically coupled to said output terminals.
 2. Apparatus as in claim 1 wherein said controllable impedance means comprises a semiconductor current control meanS having input, output and control electrodes, said input electrode being connected to said first junction, said output electrode being connected through a resistor to said second junction, and said control electrode being coupled to said source of reference potential.
 3. Apparatus as in claim 2 and further including magnetically actuatable means for varying said source of reference potential from a location outside of the body. 